The present invention relates to spinning pitch for carbon fibers and a process for its production. More particularly, it relates to spinning pitch which provides carbon fibers having high strength and high modulus of elasticity, and a process for its production.
Carbon fibers and graphite fibers have very high specific strength and specific modulus and thus are used as reinforcing materials for various composite materials for a wide range of applications including sporting goods such as fishing rods and shafts of golf clubs, medical equipments such as artificial hands and artificial legs and aerial and space navigation parts such as wings of aircrafts and doors of space shuttles.
High performance carbon fibers and graphite fibers are generally classified into polyacrylonitrile (PAN) type and pitch type. Carbon fibers and graphite fibers of pitch type are prepared usually by using coal, petroleum or the like as the starting material. As is well known, when carbonaceous material such as heavy oil, tar or pitch is heated to a temperature of from 350.degree. to 500.degree. C., small spherical particles having a particle size of from a few microns to a few hundred microns and showing optical anisotropy under a polarized light, will form in such material. When the material is further heated, such small spherical particles will grow and integrate and finally the entire material will show the optical anisotropy. This anisotropic composition is considered to be a precursor for a graphite crystal structure, wherein a high molecular weight aromatic hydrocarbon formed by the thermal polycondensation reaction of carbonaceous material is laminated and oriented in a layered fashion.
It has been proposed to use such a thermally treated product as a starting material for high performance carbon fibers of pitch type having excellent properties such as high strength and high modulus of elasticity, by melt-spinning it through spinning nozzles, followed by infusible treatment, carbonization and if necessary graphitization.
For producing spinning pitch containing a particularly large amount of an optical anisotropic phase, it is already known to produce spinning pitch by heat-treating carbonaceous material under stirring or while blowing an inert gas or the like thereinto, as disclosed in Japanese Unexamined Patent Publications No. 42924/1982 and No. 168687/1983 or to produce spinning pitch by heat-treating carbonaceous material, followed by treatment with an aromatic solvent to recover a solvent insoluble component by solvent fractionation, as disclosed in Japanese Examined Patent Publications No. 5433/1988 and No. 53317/1989.
However, such conventional spinning pitch contains a low softening point component irrespective of the type of spinning pitch. When pitch containing such a low softening point component is subjected to melt-spinning, followed by infusible treatment and carbonization to produce carbon fibers, the elastic modulus of the resulting carbon fibers tends to be inadequate due to the presence of such a low softening point component, and to supplement the deficiency in the elastic modulus, it is necessary to increase the baking temperature. If the elastic modulus is increased by increasing the baking temperature, the compression strength at 0.degree. C. of the resulting carbon fibers tends to be low, whereby it tends to be difficult to obtain high performance carbon fibers. To solve such a problem, it is conceivable to remove the low softening component by such means as solvent-extraction. However, if the low softening point component is simply removed from spinning pitch, the softening point of the spinning pitch will be high, whereby it will be necessary to increase the spinning temperature.
Under these circumstances, the present inventors have conducted extensive researches to solve such problems and as a result, have found it effective to adequately remove not only the low softening point component but also the high softening point component from spinning pitch, and they further found that with the spinning pitch having the low softening point component and the high softening point component adequately removed, the width of glass transition temperature (.DELTA.Tg) as measured by a differential scanning calorimeter is small and that the spinning pitch having a small .DELTA.Tg and a large content of an optically anisotropic phase and showing a predetermined viscosity at the spinning temperature, is capable of solving the above problems and capable of presenting high performance carbon fibers without any problems in the process. The present invention has been accomplished on the basis of these discoveries.
Further, the present inventors have found that by treating carbonaceous material having a high content of an optically anisotropic phase, with solvents having certain specific solubility parameters, it is possible to adequately remove the low softening point component and the high softening point component and to obtain spinning pitch showing certain specific physical properties. The present invention has been accomplished based also on this discovery.